A stabilized power supply unit having a current limiting function is widely used in a series regulator serving as a convenient power supply unit and a constant voltage charging apparatus for charging a battery.
FIG. 5 shows a circuit structure of a series regulator having a conventional current limiting function.
The series regulator shown in FIG. 5 is composed of a voltage control circuit 10, an output circuit 20, and a current limiting circuit 30, integrated on an IC chip.
The voltage control circuit 10 is provided with a differential amplifier Amp and voltage dividing resistors R11 and R12. The differential amplifier Amp is provided at one input terminal thereof (inverting input) with a reference voltage Vref for setting an output voltage, and at another input terminal thereof (non-inverting input) with an output feedback voltage Vfb obtained by dividing the output voltage by the voltage dividing resistors R11 and R12. The difference between the two inputs is amplified by the differential amplifier Amp, and outputted from the voltage control circuit 10 as a control voltage Vc. The differential amplifier Amp is supplied with a constant current from a constant current source 11.
The output circuit 20 has an output transistor Q21 consisting of a p-type MOS transistor (hereinafter referred to as p-type transistor) connected between a power source potential Vdd and the output terminal Po of the power supply unit. The control voltage Vc is applied to the gate of the output transistor Q21. Connected to the output terminal Po is a load Lo and a condenser Co for stabilizing the output to the load.
The current limiting circuit 30 includes a p-type current detection transistor Q31 and a detection resistor R31 connected in series in the order mentioned between the power source potential Vdd and the ground. The current limiting circuit 30 is also provided with an n-type MOS transistor (hereinafter referred to as n-type transistor) Q32 having a gate impressed with the voltage drop across the resistor R31. Constant voltage control function of the voltage control circuit 10 is regulated by the operating condition of the n-type transistor Q32.
The detection transistor Q31 is formed together with the output transistor Q21 on the same IC chip with a predetermined ratio in size less than 1 as compared with the output transistor Q21. The gate of the n-type transistor Q31 is impressed with the same control voltage Vc as the gate voltage of the output transistor Q21. As a consequence, a detection current Io′ which is practically proportional (e.g. {fraction (1/100)}) to the output current Io flowing through the output transistor Q21 flows through the n-type transistor Q31. The voltage drop across the detection resistor R31 by the detection current Io′ determines the operating condition of the n-type transistor Q32. The threshold voltage of the n-type transistor Q32 is set to the voltage that corresponds to the output current (i.e. load current) Io being a preset over-current protection level Is0. The threshold voltage is determined by the ratio of the output current Io and the detection current Io′, the resistance of the detection resistor R31, and properties of the n-type transistor Q32.
Operation of the conventional series regulator will be described with reference to FIG. 6, which shows a characteristic relationship between the output voltage Vo and output current Io of the regulator. Under normal operating condition in which the output current Io is below the limit of over-current, the voltage control circuit 10 outputs a control voltage Vc so as to equalize the output feedback voltage Vfb with the reference voltage Vref. This control voltage Vc is applied to the gate of the output transistor Q21 of the output circuit 20 to bring the output voltage Vo to a predetermined set voltage Vs. In this way, the constant voltage control of the regulator can be maintained stable at all times regardless of the magnitude of output current Io, unless the output current Io reaches the preset over-current protection level Is0.
Under such stable condition, the voltage drop by the detection resistor R31 due to the detection current Io′ does not reach the threshold voltage of the n-type transistor Q32. Hence, nothing affects the constant voltage control function of the regulator.
However, as the output current Io reaches the preset over-current protection level Is0, the voltage drop across the detection resistor R31 reaches the operating threshold voltage of the n-type transistor Q32. Thus, the n-type transistor Q32 enabled as the output current Io exceeds the preset over-current protection level Is0. In the voltage control circuit 10, current limiting operation is prioritized, so that the output voltage falls quickly, almost vertically. In this sense, this over-current protection function is a drop-type characteristic. The current level Is1 at which the output voltage fully drops down to Vo is slightly higher than the preset over-current protection level Is0, in accordance with the gain (control gain) of the current limiting regulator. The region above the level Is0 is an over-current region.
In this way, under normal condition the output voltage Vo is controlled to be at a preset voltage Vs. However, if the output current exceeds a predetermined level (over-current protection level Is0), the output current Io is automatically limited.
Hence, the output transistor Q21 must have a capability to continuously provide the maximum current Is1 in the over-current region α, exceeding the preset over-current protection level Is0. Therefore, a series regulator preferably has as small over-current region α as possible, which is ideally zero from the point of design of series regulator.
However, in general there is provided a stabilizing condenser Co at the output end of the series regulator. Then, if the over-current region a is too small, inrush current to the condenser Co will cause an oscillation at the time of startup. In other words, output transistor Q21 is perfectly conductive at the time of startup since the voltage across the condenser Co, and hence the output voltage Vo, is then zero and this causes (or tends to cause) a large inrush current to flow into the condenser. As the inrush current is detected, the current limiting circuit 30 is caused to turn off the output transistor Q21. At this stage, the output transistor Q21 is again perfectly conductive, since the output voltage Vo is still substantially zero. This permits a large inrush current to flow, thereby causing the current limiting circuit 30 to operate again. In this manner, the control of the series regulator is lost, creating an oscillation in the circuit, which hinders a smooth rise of output voltage Vo. In addition, this oscillation can disadvantageously give adverse influence (e.g. vibrations) on other components of the series regulator, and can be a source of noises to peripheral devices.
Alternatively, the current limiting circuit 30 can be of a slow-response type having a sufficient margin for oscillation. In this case, although oscillations are avoided, inrush current during a startup cannot be avoided. Therefore, it presents another problem that the condenser Co and the output transistor Q21 will be deteriorated by the inrush current.